I have a 1993 190E 2.3 8V that started a consistently high idle suddenly a few months ago (circa 2,000 rpm free idle once warmed up, in P or N, 1,000rpm in gear). I parked it due to high braking effort needed on the 20 stoplights between home and work. Here are my observations so far.

• Cold idle around 1,200rpm when cold, then it as it gets up to temperature the idle increases to 2,000rpm.

• I can shut the car off, immediately restart it, and the idle is around 1,000 rpm.

• OVP was checked by Indy this past week and replaced with a new one, no change in idle behavior.

• I can push back the linkage slightly to change states of the cut-off switch for idle, and the idle changes slightly so I feel that switch is functioning.

• Measuring pin 3 to ground on XD11 connector gets near battery voltage (about 12 volts compared to around 13V for the batter) - means its running a consistently rich mixture from what I can glean from the myriads of discussions I have found

• I had to replace the original catalytic converter last summer

• O2 sensor hasn't been replaced in the time I have owned the car (got it at 121K miles and it has 188K now)

• I replaced the spark plugs yesterday to see if it helped, old spark plugs were worn but didn't look bad - pretty good uniform brown color with a little blackening that wasn't objectionable.

I'm stumped and my Indy is stumped - do I need to replace the O2 sensor so that the system isn't running it rich and increasing airflow to compensate for the extra fuel, or is the O2 reading a rich mixture properly and I just need to adjust the mixture on the tower on the airhorn, or am I completely misunderstanding whats going on here and there's another issue?